1. Field of the Invention
The present invention relates to an intermittent web feeding apparatus and, more specifically, to an intermittent web feeding apparatus employing a servomotor for intermittently feeding a fixed length of a continuous web of material drawn out from a roll of web material during one feeding cycle and which is capable of intermittently feeding the web under tensioned conditions at a high rate of feed speed.
2. Description of the Prior Art
Continuous webs, such as continuous thin films, of soft synthetic resins are widely used as packing material due to their excellent impermeability and flexibility. A bag forming and packing machine generally designated as a pillow packer is practically used as an automatic packing machine which packs articles in packing bags formed on such a web of material. Typically, this packing machine is of the vertical type capable of automatically carrying out a series of processes including intermittently feeding a web of material through means of a fixed package forming length during one feeding cycle, forming a bag with the web of material of the fixed package forming length, supplying an article in the bag, and sealing the bag.
The present invention provides an intermittent web feeding apparatus for such a vertical packing machine. In order to facilitate the understanding of the present invention, the general mode of the packing operation of the vertical packing machine and the mode of the intermittent web feeding operation for the vertical packing machine will be described with reference to FIGS. 1a, 1b and 2 prior to the description of the present invention.
A continuous web of material 2 unwound from a roll 1 of web material (one among two rolls 1 of web material is a preparatory roll of web material for the next use) travels via guide rollers 3 and 4 disposed at appropriate positions, respectively, and is fed by means of a pair of feed rollers 5. The feed rollers 5 rotate intermittently so as to feed the web of material 2 intermittently a fixed length during one feed cycle. A packing machine has a vertical guide tube 6 serving as a chute for feeding articles, and a pair of rotary feeding units 10 disposed upon opposite sides of the guide tube 6, respectively. The rotary feeding units 10 are positively driven intermittently in synchronism with the feed rollers 5 and at the same circumferential speed as that of the feed rollers by means of a driving mechanism which will be described later. The rotary feeding units 10 feed the web of material a fixed distance and form the web of material 2 into the shape of a bag. Preferably, each rotary feeding unit 10 is a belt conveyor comprising an upper pulley 12, a lower pulley 11 and an endless belt 13 extending between the upper pulley 12 and the lower pulley 11.
The web of material 2 unwound from the roll 1 of web material by means of the feed rollers 5 is folded into a tubular shape by means of a former 7 surrounding the upper portion of the guide tube 6. As the tubular web 2A is moved intermittently along the outer circumference of the guide tube 6 through a fixed distance corresponding to the packing length of the articles, the opposite side edges 2a of the tubular web 2A are heat-sealed by means of the heat-sealing members 9 of a center sealing device 8 disposed adjacent to the outer circumference of the guide tube 6 so as to form a packing tube 2B. Then, the packing tube 2B is moved downwardly along the guide tube 6 by means of the rotary feeding units 10 (FIG. 1a).
During the interruption of the intermittent feeding of the packing tube 2B, an article W to be packed is supplied from a hopper, not shown, through the guide tube 6 into the end-sealed portion 2b of the packing tube 2B. The lower end of the end-sealed portion 2b of the packing tube 2B has been previously sealed. Then, the packing tube 2B is moved further downwardly through by a fixed distance corresponding to the packing length of the articles along the guide tube 6, and then the heat-sealing members 15 and the cutting knife 16 of an end sealing device 14 are actuated so as to heat-seal the upper end of the end-sealed portion 2b, so that the article W is sealed in a pillow bag 2c. The same procedural cycle including unwinding the web of material 2 from the roll 1 of web material, intermittently feeding the web of material 2, forming the packing tube 2B, supplying the article W into the packing tube 2B, and end-sealing and cutting the end-sealed portion 2b is repeated for the intermittent bag-forming and packing operation.
In this vertical packing machine, the feed rollers 5 positively withdraw the web of material 2 from the roll 1 of web material, while the rotary feeding units 10 positively feed the packing tube 2B. The feed rollers 5 and the rotary feeding units 10 are operated synchronously for the intermittent feeding of the web of material 2 and the packing tube 2B. However, each time the intermittent feeding of the web of material 2 and the packing tube 2B is interrupted, the web of material 2 tends to slacken between the feed rollers 5 and the bag former 7. The slackening of the web of material 2 results in the zigzag movement of the web of material 2, which makes the smooth feeding of the web of material 2 impossible and causes deformation and irregularity in the packages resulting in the faulty sealing of the packages.
The applicant of the present invention previously proposed "A Web Tensioning Method" in Japanese Patent Application No. 59-77010, as a means for preventing the slackening of the web of material and for achieving regular intermittent feeding of the web of material, and applied the proposed method to a practical intermittent web feeding apparatus. This known intermittent web feeding apparatus combined with the above-mentioned vertical packing machine has a single general purpose motor for driving the transmission systems of the feed rollers and the packing tube feeding unit in unison, electromagnetic clutches provided in the transmission systems, and a one-way clutch interlocking one of the electromagnetic clutches and the packing tube feeding unit. In the starting mode, the revolving speed of the motor is controlled and the electromagnetic clutches are controlled so as to start the feeding rollers and the packing tube feeding unit simultaneously. In the stopping mode, since the one-way clutch is provided, the packing tube feeding unit is stopped with a delay after the feed rollers have been stopped, and thereby a moderate tension is applied to the web of material when the web feeding operation is interrupted.
This method proposed by the applicant of the present invention prevents the slackening of the web of material and always keeps the web of material tight under a moderate tension for smooth intermittent web feeding, and hence the intermittent web feeding apparatus employing this method is sufficiently capable of achieving a low-speed bag-forming and packing operation. However, since the response speed, namely, the speed of response to control signals provided by a control circuit, of the electromagnetic clutches in starting and stopping the feed rollers and the packing tube feeding unit is limited, this intermittent web feeding apparatus is incapable of achieving a high-speed intermittent web feeding operation, which has been agressively desired by the users. The employment of dog clutches instead of friction clutches will enhance the processing speed to some extent. However, since dog clutches produce shocks and noise when engaged or disengaged, thereby increasing the frequency of operational difficulties, and hence dog clutches are not desirable. When mechanical clutches regardless of type are employed, the operating speed of the apparatus is limited and the intermittent web feeding apparatus is unable to meet the requirements of the industry.